The invention is related to means for setting the beam currents in video cameras, and particularly to a method and simple circuit thereof for automatically setting the beam current of a pickup tube to a greater value, i.e., 200% of typical, which precludes blooming in the presence of highlights of up to twice the usual level.
Typically, a pickup tube such as a plumbicon (trademark of N. V. Philips), Saticon (trademark of Hitachi Denshi, Ltd.), etc., is operated at 300 nanoamps (nA) of beam signal current in the green channel. Video camera manufacturers generally set the beam current of such a tube at a value 100% larger, or 600 nA, whereby the tube can handle up to twice as much light in a scene without experiencing the picture wash-out effect known as "blooming". On the other hand, setting the beam current to a value which is too high will cause defocussing effects.
Thus, to achieve the proper beam current setting, a typical prior art approach used in manual cameras during the setup procedure, is to open the iris of the camera to obtain 100% picture, with the gain of the respective channel set to normal. The f/stop number of the iris is then opened one f/stop larger, which guarantees twice the amount of incoming light. Then the beam current is adjusted whereby the tube just handles the doubled amount of light. The beam current is then set at 200% of the typical beam current.
A further approach for setting the beam current in the manual setup of a camera is to adjust the normal gain of the channel to a -6 dh gain, i.e., exactly halve the normal gain of the channel. The iris is then opened to provide 100% of the output. Since the gain has been halved, there is 200% incoming light to the tube. Now the beam current is readjusted such that there is no blooming effect, whereby the beam current thus is set at 200% of the typical level.
Such techniques may be employed in the manual setup of a camera, where the iris control, etc., is available. However, the techniques are impractical for use with video cameras having sophisticated automatic setup systems, particularly in systems employing a diascope test pattern for automatic setup. In such systems, there presently is no practical way for providing 200% of incoming light to the tube. An iris may be built into the diascope, whereby the automatic setup system may open the iris by one f/stop in the course of setting the associated beam. Or the system may include a 2 to 1 reduction filter which might be switched out by the automatic setup system to double the light intensity. However, such systems are mechanical, relatively complicated and correspondingly expensive to implement.
Another approach may be to fabricate the diascope pattern with only 50% transparency, with a few sections in it with 100% transparency. Thus, with 100% of light passing through the diascope, the few sections would pass 200% of light, which then may be used by the automatic setup system to provide a 200% beam setting. This technique is difficult to apply in practice because of the difficulty of making the diascope with the required transparencies.